Preheat oven to 425°. Toss cauliflower florets, onion and potatoes on a parchment lined baking sheet with onion, thyme, and olive oil; season with 1 tsp salt and pepper to taste.
Put the cauliflower and potatoes in the oven. Roast, tossing once, until they are tender and golden, 20-25 minutes.
While the cauliflower and potatoes are roasting, mix together the honey, mustard and garlic. Spread on top of the salmon and season with salt, to taste.
After 20-25 minutes, remove the sheet tray from the over and push the cauliflower and potatoes to the outer edges of the baking sheet, leaving room in the center for the salmon.
Place the salmon on the sheet tray, and sprinkle the cauliflower and potatoes with Parmesan.
Roast 10-15 minutes longer until the salmon is cooked and the veggies are golden.
Whisk together the lemon juice, olive oil and salt. Drizzle over the top of your salmon, cauliflower & potatoes.